Comparisons of plankton at 59°N and 37°N during two Lagrangian drift experiments in the North Atlantic in June and July 1996

Abstract

Data are summarised for two Lagrangian experiments in the North Atlantic in early summer 1996. At 59°N 20°W, plankton dynamics was studied in an SF 6 tracer release experiment within a mesoscale eddy over a 9-day period. At 37°N 20°W, a second experiment followed a drifting buoy for 7 days. The data obtained in these two experiments have been averaged for 3 depth strata; the euphotic zone, the surface mixed layer (SML), and the seasonal thermocline immediately beneath the surface mixed layer. At 59°N, the euphotic zone was only marginally deeper than the SML, but at 37°N the SML was ca 30 m and the euphotic depth was ca 110 m. At 37°N, nutrient concentrations in the SML were low but significant new production occurred in the thermocline because of light penetration into the nutricline. The particulate organic carbon (POC) concentration of the SML at 59°N was 13–15 μmol C kg −1, but at 37°N POC concentrations were 4 μmol C kg −1. These POC measurements include biota and detritus. As a way of investigating latitudinal differences in the plankton communities, estimates have been made of the carbon and nitrogen content of phytoplankton, bacterioplankton, microzooplankton and mesozooplankton. At both 59°N and 37°N, phytoplankton was the largest component, accounting for ca 50% of the planktonic biomass in the SML. At 59°N, microzooplankton was 16% of the planktonic carbon, but at 37°N this reduced to 8% of the total. Mesozooplankton was a relatively constant proportion (ca 20%) of the planktonic carbon in the SML at both 59°N and 37°N. Bacterioplankton was 14% of the biomass at 59°N, increasing to 24% in the microbial loop-dominated system at 37°N. Mean carbon fixation rate in the oligotrophic southern station was 24% of that at the north, with more carbon fixation below the SML at 37°N than at 59°N. Respiration rates showed little variation with latitude, and the rates at 37°N were 80% of those at 59°N. Nitrate and ammonium uptake rates were very low in the oligotrophic conditions in the SML at 37°N, but nitrate uptake in the euphotic zone was comparable to that at 59°N. Ammonium uptake by phytoplankton was also significantly greater at 37°N, in both the euphotic zone and thermocline, but uptake in the SML was only 20% of that in the SML at 59°N.